Along with the exploitation of large quantity of petroleum resources, the world petroleum resources are expected to be depleted within future 50-100 years. In recent years, frequent fluctuations have appeared in petroleum price, and the rapid development in scientific research and industrial exploration of synthesis of chemical products and liquid fuels, refined from petroleum in the past, by utilizing syngas has been realized.
The syngas refers to mixed gas taking H2 and CO as main components, and is widely applied to synthesis of fine chemical products, industrial synthesis of ammonia, synthesis of methanol, fuel cells, power generation by gas turbines and the like. In 2010, China consumed 2.4 billion tons of coal, wherein about 36% was used for preparing the syngas by gasification and then used for indirect synthesis of the chemical products and the liquid fuels. Meanwhile, technologies for producing the syngas by gasification of biomass, municipal solid waste and industrial wastes are focal points and hotspots in the development directions of utilization of those energy. However, the H2/CO ratio in the crude syngas obtained by gasification is generally low (usually 0.5-1), the crude syngas contains a small quantity of tar, pollutants containing S, N and Cl (such as HCl, H2S, HCN, NH3, CS2, COS and the like) and gaseous hydrocarbons (such as CH4, C2H2, C2H4 and the like), and these hydrocarbons may cause carbon deposition on catalysts and cause deactivation of catalysts, meanwhile consume the compression work and reduce the efficiency of the system, so that the quality requirements of common synthesis processes on the syngas can only be met by selectively converting or removing the above components before synthesis, and a purification and conditioning technology of the crude syngas has great application prospects.
The purification and conditioning process of the syngas generally comprises H2/CO adjustment, separation of carbon dioxide, removal of tar, removal of S/Cl/N pollutants and other processes. At present, in the above process, all the steps are completed in different reactors respectively in a modular stacking way in general. Although there have been intensively studied and have certain industrial applications on those modular stacking way. The current modular stacking step-by-step purification and conditioning process also has several obvious disadvantages: 1. the process flow is long, and the investment cost is high. The purification and conditioning process is performed in different reactors step by step, so investment cost in space, reaction equipment and auxiliary facilities is increased. 2. All the process steps in purification and conditioning interfere with each other, and the difficulty in operation and design is large. For example, it is difficult to completely remove CO2 at high temperature, and although the removal effect at low temperature is better, the follow-up sulfur removal and chlorine removal effect will be affected; the crude syngas generally simultaneously contains the pollutants containing S and Cl, but the pollutants containing Cl can cause the deactivation of a desulfurization catalyst; and the S and the Cl pollutants can also deactivate a tar cracking catalyst, if the S and the Cl are firstly removed and then the tar is subjected to catalytic cracking, as the operating temperature of a desulfurization and dechlorination agent is lower than that of a tar cracking catalyst, the temperature of the crude syngas needs to be adjusted, and the heat loss of the system is further caused. These problems cause a certain difficulty to design of the process and the safety operation of the system.
A deep purification method of syngas is disclosed in Patent CN 101224871, in which the crude syngas passes through a fine hydrolysis catalyst, a fine desulfurization agent, a protective agent, a deoxidization agent and a dearsenication agent so that the total sulfur (H2S+COS) is less than 0.01 ppm, namely 10 ppb, the chlorine is less than 0.01 ppm, namely 10 ppb, carbonyl metal compounds are less than 0.02 ppm, namely 20 ppb, oxygen is less than 1 ppm, arsenic is less than 0.02 ppm, namely 20 ppb, and further the purpose of deep purification is achieved. The method has more process steps and complex process. A liquid nitrogen washing device for purifying syngas is provided in Patent CN 201729816U. A leading-out position after cooling of a first raw material gas cooler and an entering position for further entering a second cooler are arranged to enable a temperature splitting point of the syngas to be far away from a dew point of a methane fraction; and furthermore, the leading-out position from the second cooler to a raw material gas separator is arranged to enable the temperature to be lower than the dew point of the methane fraction, and CH4 can be effectively separated and recovered. The method can not simultaneously purify S, Cl and other pollutants in the syngas. A method for synchronously making syngas and metal zinc is disclosed in Patent CN 101157443. In molten salts reactor, a carbonate molten salts is taken as a reaction medium, oxidation and reduction reactions are performed through methane and zinc oxide powder, and mixed gas of hydrogen and carbon monoxide as well as the metal zinc can be synchronously generated. The method can not synchronously adjust the H2/CO and crack the tar.
All the above patents do not refer to purification methods of the crude syngas, which selectively convert the hydrocarbons in the crude syngas in the presence of oxygen, then utilize the molten salts to absorb the pollutants in the crude syngas and simultaneously adjust the H2/CO ratio.